The present invention relates to a brake-pressure control device for a road vehicle with electrohydraulic multi-circuit brake system.
A conventional brake-pressure control device is described in DE 43 35 769 C1. This known brake-pressure control device comprises hydraulic servo-cylinders which are individually assigned to the wheel brakes, can be driven by electric motor and permit, in a single stroke of their piston, the maximum brake pressure in the connected wheel brake for which the brake is designed to be increased and decreased. Under the control of the electronic output signals of an electronic control unit, the device enables at least the following functions to be implemented:
(a) Control of the spot braking mode in accordance with desired-value signals which can be generated by the driver by a desired-value setting unit which can be activated by the driver and are characteristic of the anticipated value of the deceleration of the vehicle, including control of the front-axle/rear-axle braking force distribution; PA1 (b) If appropriate, automatic triggering of full braking if the activation behavior of the driver, which can be detected from a chronological processing of sensor output signals of the desired-value setting unit, signals the driver's desire for high deceleration of the vehicle; PA1 (c) Anti-lock braking control by automatically controlled brake-pressure modulation; PA1 (d) Traction control by automatic activation of the wheel brake for each vehicle wheel which tends to spin; PA1 (e) Vehicle movement control by automatically controlled increase in brake slip at one or more of the wheels of the vehicle; and PA1 (f) If appropriate, a vehicle spacing control for backed-up traffic by automatic activation of the brake system as a function of output signals of a sensor system which detects the distance from a vehicle which is travelling in front.
The desired-value setting unit comprises a static single-circuit master cylinder which can be activated directly by a customary brake pedal, i.e. without intermediate connection of a brake booster, and to whose output pressure space the front-wheel brakes are connected via, in each case, one change-over valve which is constructed as a 2/2-way solenoid valve. The switched position of the valves which is assumed when their switching magnets are excited is a blocking position in which the front-wheel brakes are blocked off from the output pressure space of the single-circuit master cylinder and in the flow position, which is assumed as the normal position, brake fluid can be displaced directly into the front-wheel brakes by activating the master cylinder. Thereby, in the event of a failure of the vehicle electrical system, emergency braking is possible by activating the front-wheel brakes, to achieve a relatively high deceleration of the vehicle of up to 0.4 g (g=9.81 ms.sup.-2) which is higher than the legally required minimum deceleration.
So that, with the known brake-pressure control device, a pedal travel/braking force characteristic of the brake-pressure control device which is favorable, in terms of ergonomic criteria, for a good metering of the brake pressure generating procedure which is to be initiated by the driver in the case of spot braking can be achieved, the feeding of brake fluid into the wheel brake cylinders takes place during spot braking as a result of the combined effect of the master cylinder and of the brake-pressure servo-cylinders. For example, the feeding takes place by virtue of the actuation travel of the pistons of these servo-cylinders for whose monitoring position transmitters which are individually assigned to the servo-cylinders are provided is adjusted in accordance with the actuation travel of the piston of the master cylinder which can also be detected by a pedal-position sensor. A defined ratio of the quantity of brake fluid which is fed into the front-wheel brakes by the master cylinder to the quantity of brake fluid which can be fed into the front-wheel brakes by the servo-cylinders is thereby obtained. In the possible situations of automatic activation of the front-wheel brakes which takes place for the methods of adjustment and control mentioned above, the brakes are blocked off from the master brake cylinder by actuation of the change-over valves. Thereafter, the brake-pressure control takes place solely by actuation, which is appropriate for control, of the brake-pressure servo-cylinders both of the front-wheel brakes and of the rear-wheel brakes of the vehicle, at the rear-wheel brakes also in the case of spot braking.
This function of the known brake-pressure control device has the effect that, when the anti-lock braking control responds, the pedal reaction which is intended, as it were, to provide the driver with the acknowledgement of the functional state of the brake system, changes drastically, because the brake pedal becomes, so to speak, "hard" in that it can no longer be moved on, and the further metering of the brake pressure which is used for building up the brake pressure again is made difficult. This is disadvantageous in particular if the braking takes place on a section of roadway with a very low coefficient of adhesion between the roadway and the wheels of the vehicle. Under this condition, a particularly short pedal travel results, i.e. a sudden change in the pedal reaction which suggests to the driver, in none too seldom cases, the impression of a defect in the brake system. That is, a distraction arises from which incorrect driving behavior, and thus a potentially hazardous situation, can well result. Brake activation situations which are similarly capable of misinterpretation and, moreover, also adversely affect comfortable operation of the brake system can arise if the driver wishes to brake manually during a vehicle-movement or vehicle-spacing control phase and, when doing so, is initially confronted with a "hard" brake pedal because the change-over valves are still closed. The same applies correspondingly to the case in which the vehicle is equipped with a front-axle drive and the driver wishes to initiate braking in the course of a traction control phase during which the change-over valves of the front-wheel brakes are likewise closed.
In addition, the maximum capacity of the output pressure spaces of the servo-cylinders of the front-wheel brakes must be greater than the maximum capacity of the connected wheel brakes. In other words, starting from a normal position of the servo-cylinder pistons in which the volume of the servo-cylinder output pressure spaces corresponds to the maximum absorption volume of the connected wheel brakes, in the event of anti-lock braking control only starting at a very high brake pressure, complete elimination of the brake pressure is nonetheless possible which, in this respect, requires an increased overall length of the servo-cylinders of the front-wheel brakes.
A brake system with electric braking-force actuators is also described in DE 34 10 006 A1. Furthermore, this brake system also has a master brake cylinder which applies brake pressure to hydraulic wheel brake cylinders of the front axle. The hydraulic generation of brake pressure is configured as a fallback level. The electric braking-force actuators are regulated by a control device which, for this purpose, is fed a signal relating to the activation of the brake pedal and signals relating to the wheel speeds of the individual wheels. The function of an ABS system or of a traction control system can also be integrated into the control unit.
An object of the present invention is, therefore, to provide an improved brake-pressure control device such that, in an overwhelming number of braking situations which are controlled by the driver by activating the single-circuit master cylinder, the reaction force which can be felt at the brake pedal as a function of the pedal travel is a reliable measure of the anticipated value of the deceleration of the vehicle and a pedal travel/brake pressure characteristic is achieved which also helps to avoid distractions relating to the operational capability of the brake system.
This object has been achieved according to the present invention by providing a single-circuit master cylinder which is configured such that the volume of brake fluid displaceable from an output pressure space thereof as a result of displacement of a piston thereof by a maximum stroke s.sub.max is substantially greater than a capacity of the wheel brakes of the front-axle brake circuit which has to be displaced into the brakes to achieve a defined pressure necessary for minimum deceleration. The front-wheel brakes are configured to be also blocked off from the single-circuit master cylinder in the event of electrically controlled spot braking, and a maximum capacity of a output pressure spaces of the brake-pressure servo-cylinders of the front-wheel brakes is limited to a volume capable of being displaced into the respectively connected front-wheel brake to achieve a maximum brake pressure. A storage element into which brake fluid can be displaced from the single-circuit master cylinder counter to an increasing reaction force is connected to a pressure outlet of the single-circuit master cylinder of the desired-value setting unit, the maximum capacity of the storage element corresponding at maximum to an additional amount by which the volume of brake fluid which is displaceable from the master cylinder is greater than the volume of brake fluid which is displaceable into the front-wheel brakes in emergency operating mode to achieve the defined minimum deceleration.
Accordingly, the single-circuit master cylinder is dimensioned such that the total volume of brake fluid which can be displaced from the output pressure space of the master cylinder by displacing the piston of the master cylinder by the maximum stroke s.sub.max which can be prescribed by physical means is significantly, preferably by a defined fraction, greater than the volume of that quantity of brake fluid which has to be displaced into the front-wheel brakes in order to achieve in the brakes a defined brake pressure which is provided for emergency braking situations according to the configuration of the brakes. In combination therewith, also in the event of electrically controlled spot braking, the front-wheel brakes are blocked off from the single-circuit master cylinder by actuation of the change-over valves. That is, the brake pressure build-up takes place solely by activation of the brake-pressure servo-cylinders. It is thus possible, as further provided, for the maximum capacity of the output pressure spaces of the brake-pressure servo-cylinders to be limited to that volume which has to be capable of being displaced into the respectively connected wheel brake in order to achieve a maximum brake pressure in the wheel brake, and this corresponds to the lowest possible spatial requirement of the brake-pressure servo-cylinders.
Furthermore, a storage element is connected to the pressure outlet of the single-circuit master cylinder of the desired-value setting unit, into which storage element brake fluid can be displaced counter to an increasing reaction force by activating the single-circuit master cylinder. Thereby, a displacement of the master cylinder piston, and thus an ergonomically favorable travel/force characteristic curve of the brake-pressure control device can be attained even if the front-wheel brakes are blocked off from the single-circuit master cylinder.
Finally, in combination with the foregoing aspect of the present invention, the maximum capacity of the storage element can correspond to, at maximum, the additional amount by which the volume of brake fluid which can be displaced from the master cylinder is greater than the capacity of the front-wheel brakes which is necessary in emergency braking situations to achieve a defined minimum deceleration. As a result, it is ensured that a build-up in brake pressure which is necessary for a sufficient deceleration of the vehicle of, for example, 0.4 g remains possible solely by activating the single-circuit master cylinder, in an emergency braking mode of the brake-pressure control device which has to be possible even in the event of a failure of the vehicle's electrical system.
A favorable relation between pedal travel in the emergency braking mode and the capability of metering in the spot braking mode is obtained if the maximum capacity of the storage element is between 30 and 60%, preferably about 50%, of the total volume of brake fluid which can be displaced in total into the front-wheel brakes.
A storage element which is suitable for these relations can be easily realized as a piston-spring storage device. In particular, for the emergency braking mode of the brake-pressure control device, the storage element can advantageously be blocked off from the single-circuit master cylinder. Thereby, the entire volume of brake fluid displaceable therefrom is available for building up the brake pressure in the front-wheel brakes.
It is easily possible to block off the storage element from the master cylinder automatically, with any desired appropriate configuration of the storage element, in the event of a failure of the vehicle's electrical system. This can be effected by connecting a solenoid valve between the storage element and the master cylinder. The excited position of the solenoid valve is its flow position, and its normal position, which is spring-centered, is its blocking position.
Provided that the storage element is constructed as a piston-spring storage device in which the storage spring is arranged in a spring chamber which is filled with brake fluid and can be communicatingly connected to the brake-fluid reservoir vessel, a functionally corresponding solenoid valve can be connected between the reservoir vessel and the spring chamber.
As an alternative to a "passive" piston-spring storage device, a hydraulic cylinder which can be driven by an electric motor and has a piston whose position is monitored can constitute the storage element. The drive of the cylinder preferably is not self-locking but instead provided with a mechanical arresting brake which is automatically active in the de-energized case. With such a storage element, in principle any desired pedal travel/reaction force characteristic curves can be obtained by appropriate electrical actuation of the drive. It is also possible, in the event of anti-lock braking control, to initiate a reaction behavior of the master cylinder which intelligibly signals the activation of anti-lock braking control to the driver.
So that excessively large pedal activation travels do not have to be tolerated in emergency braking mode, it is particularly advantageous if, for emergency braking, the single-circuit master cylinder can be adjusted to a ratio of its displacement volume related to the piston stroke which is increased in comparison with the spot braking mode. It is particularly advantageous for safety reasons to adjust the master cylinder automatically, for example with path control, to the displacement volume/stroke ratio which is increased for the emergency braking mode.
Alternative configurations of the single-circuit master cylinder which are suitable for the foregoing purposes are disclosed below and enable activation, in each case after a minimum stroke of the master cylinder piston which can be activated by the brake pedal, of an additional piston face and as a result increasing the volume of brake fluid which can be displaced from the master cylinder per unit of the stroke.
Configuring the master cylinder such that a prestressed compression spring, whose prestress is smaller than that of the restoring spring of the single-circuit master cylinder, is arranged in the radially inner annular space of the piston arrangement is particularly advantageous in this context, in which configuration a steady transition of the force/travel relation, which is decisive for the spot braking mode, to the force/travel relation which is valid in the emergency braking mode can be achieved by appropriate matching of a spring, which acts between two piston elements, to the reaction force which is conditioned by the brake pressure.